This invention relates to heat transfer and more particularly to a heat transfer system using a nanoporous membrane to provide capillary pressure for high heat transport and decoupled from fluidic delivery.
Increasing power densities in high performance systems have led to significant demands for advanced thermal management technologies. Evaporative microfluidic cooling strategies have received significant interest to address the stringent heat flux requirements in high performance systems while significantly reducing size, weight, and power consumption (SWaP). In particular, flow boiling in microchannels has been an active research area [3-5, 24-27] where typical dissipated heat fluxes of q″=200 W/cm2 and heat transfer coefficients of h=10 W/cm2K using water have been demonstrated for exit qualities below 20% [24]. While heat transfer can be improved when in the annular flow regime with much higher exit quality, challenges exist with flow instabilities [3-5, 25-27]. Despite incorporating surface features and using various water mixtures allowing q″=600 W/cm2 and h=8.8 W/cm2K, the performance remains fundamentally limited by instabilities [28]. Accordingly, flow restrictors have been proposed [29], but lead to significant increases in pumping power requirements.
Recent developments in thermal ground planes have utilized micro/nanostructured wicks, including sintered copper mesh coated with carbon nanotubes [30], titanium nanopillars [31], and oxidized copper microposts [32], to develop high flux evaporators. A typical value of q″=550 W/cm2 and h=15.4 W/cm2K over a heated area of 5×5 mm2 was demonstrated with an evaporator area of 2×2 cm2 [30]. However, the performance of such wick is fundamentally limited due to the coupling between the capillary pressure generated by the wick and the liquid transport through the wick. To decrease the transport distance within the wick, liquid supply arteries [33] or bi-porosity [34, 35] have been introduced, and as a result, q″=380 W/cm2 with h=20 W/cm2K was demonstrated [33]. Even in this configuration, the liquid transport remains in the wick, and limits the maximum heat flux. Meanwhile, to achieve the flow rates required for higher heat fluxes, the height of the wick structure should be >100 μm, which increases the thermal resistance.
A recent study using evaporation through a nanoporous membrane, similar to our concept disclosed herein, demonstrated a maximum q″˜600 W/cm2 with h=9.4 W/cm2K [36]. However, the configuration required heat conducting through a liquid layer that limited the thermal resistance, and active pumping, instead of capillarity, to drive the liquid to the membrane, which increased the power consumption.